There are known steam generators of fast-neutron nuclear reactors cooled by a liquid metal, which comprise a bundle consisting of a set of parallel straight tubes fastened at their ends in tube plates parallel to one another and fixed to the outer casing of the steam generator. The tube plates delimit, within the casing of the steam generator, three successive zones of which the first forms a water box, the second a zone for the circulation of the heat-exchange liquid metal in contact with the outer surface of the tubes of the bundle, and the third a steam collector.
The tube plates are pierced over their entire thickness with holes, in the region of each of which the end of a tube is fastened by welding to a connecting branch of tubular shape which projects relative to the inner face of the tube plate, this connecting branch generally being designated by the term "nipple" in the relevant art.
Each of the tubes of the bundle of the heat exchanger is fastened at one of its ends to a first branch of a first tube plate and at its other end to a second branch of a second tube plate, the inside diameter of the second branch and of the corresponding orifice of the second tube plate being substantially larger than the outside diameter of the tube which is connected to the second branch by means of a junction zone of frustoconical shape.
Each of the tubes thus makes the junction between the water box and the steam collector, the feed water of the steam generator, distributed throughout the set of tubes of the bundle at the outlet of the water box, subsequently being heated and evaporated within the tubes as a result of thermal contact with the liquid heat-exchange metal. The steam formed in the tubes is thereafter recovered in the steam collector.
The hot liquid sodium is generally conveyed to the upper part of the steam generator in the vicinity of the tube plate delimiting the steam collector; the liquid sodium subsequently circulates vertically on the outside of the tubes of the bundle.
Should one of the tubes have a crack causing a leak, this leak must be detected very quickly, so that the operation of the steam generator can be stopped and its emptying carried out before the reaction between the liquid sodium and the water flowing off via the leak reaches such a stage that the steam generator may be damaged and this reaction no longer controlled by the safety devices provided on the steam generator and ensuring the damping of pressure waves in the sodium.
A leak in the region of one of the tubes of the steam generator can be disclosed either by detection of the hydrogen forming as a result of the reaction of the water with the sodium or by acoustic detection of the noise of the reaction.
After the steam generator has been emptied completely, it is necessary to carry out a repair on the defective tube having a leak, either by plugging or by the replacement of this defective tube, in order to make it possible for the steam generator to be put back into operation.
To effect the plugging of a defective tube of a steam generator, various techniques, such as those described, for example, in the patents FR-A-2,524,609 and FR-A-2,560,962, are available.
In FR-A-2,524,609, it is proposed to carry out the plugging of the tube by means of an automatic displacement of a shutter placed inside the tube, so as to obtain the shutting off of this tube, under the effect of the pressure difference accompanying the occurrence of the leak and the succeeding reaction between the sodium and water. This plugging of the tube makes it possible to prevent the leak from having harmful consequences by separating the pressurized water or steam from the sodium safely and immediately.
FR-A-2,560,962 recommends plugging by the installation of a flexible sleeve in an axial arrangement inside the tube, this sleeve being fastened by mechanical keying or by blast welding.
In actual fact, the plugging techniques employing expanding sleeves, shutters or the fastening of a shutoff member by expansion, blast welding or keying are no longer adopted in operations for the maintenance or repair of steam generators using liquid sodium as a primary fluid. Indeed, it is necessary to guarantee that there is absolute sealing between the water and the liquid sodium during operation. None of the abovementioned techniques makes it possible to guarantee such absolute sealing.
In fact, the operation of a steam generator of a fast-neutron nuclear reactor results in very high temperatures of the heat-exchange fluid ensuring the heating and evaporation of the water, in extremely rapid transient temperature effects and in thermal shocks attributable to temperature variations, which can be very great, of the liquid metal constituting the heat-exchange fluid.
Moreover, the proportion of tubes shut off during successive repair operations on a steam generator must not exceed a particular percentage of the tubes as a whole.
It is generally considered that this percentage must be between 10 and 15% of the tubes.
The disadvantage of the technique of plugging the tubes is that it causes a loss of power of the steam generator and introduces a temperature asymmetry into the bundle of tubes, with the result that the other tubes of the bundle experience increased thermomechanical stresses.
Furthermore, for safety reasons, it is not possible to place plugs o the outer walls of the tube plates of a steam generator, i.e., those facing the water box or facing the steam collector. In fact, in this case, hydrogen detection or acoustic detection making it possible to reveal the occurrence of a leak of water or steam into the sodium cannot be conducted reliably and quickly. Belated detection of a water leak into the sodium has very serious disadvantages as mentioned above.
It is inadvisable to make welds on the tube plates which are liable to induce welding stresses in these plates and make it necessary to relieve the tube plates of stress after welding and to carry out checking procedures which are difficult to perform.
The plugs for shutting off the tubes must therefore be placed on the inner walls of the tube plates, i.e., the walls facing the zone of the steam generator containing the tube bundle. The tube plates have thicknesses which can be relatively large, for example of the order of 400 mm, and therefore the plugs have to be placed on walls located at considerable distances from the outer access faces of the tube plates.
Should the cutting and extraction of the defective tube be carried out before the plugging of the corresponding orifices of the tube plates is executed, the thermohydraulic behavior of the steam generator is modified inasmuch as the extraction of the defective tube modifies the circulation of the cooling fluid in the hydraulic channel located between the tubes adjacent to the defective tube.
Should plugging be carried out without the extraction of the defective tube from the steam generator, this tube is liable to dilate and experience buckling deformation during the operation of the nuclear reactor. The defective tube can come into contact with the adjacent tubes and cause damage to these tubes by friction under the effect of the vibrations generated by the flow of the heat-exchange fluid.